Method of forming multilayered topcoat film

ABSTRACT

Disclosed is a 3-coat 2-bake method of forming a multilayered topcoat film, comprising applying a first coating composition (A) and a second coating composition (B) to a substrate, thermally curing the two compositions, and applying and thermally curing a clear coating composition (C); 
     the first coating composition (A) being an organic solvent-based colored coating composition; 
     the second coating composition (B) being an organic solvent-based coating composition comprising an acrylic resin (b-1) having at least two side chains of different lengths, each of which is bonded to the main chain and has at least one hydroxyl group, a polyepoxide (b-2) and a crosslinking agent (b-3); and 
     the clear coating composition (C) being a powder coating composition (C-1) or an organic solvent-based coating composition (C-2) comprising a hydroxyl- and carboxyl-containing resin (c-2 a ) and a polyepoxide (c-2 b ). The method of the present invention is capable of forming a multilayered topcoat film excellent in intercoat adhesion, finish properties and film performance.

TECHNICAL FIELD

The present invention relates to a novel method of forming amultilayered topcoat film.

BACKGROUND ART

A 3-coat 2-bake method of forming a multilayered topcoat film on anautomotive outer plate or the like is known, which comprises applying afirst coating composition and a second coating composition to asubstrate usually coated with an undercoat, such as a cationicelectrodeposition coat, and an intermediate coat, thermally curing thefirst and second coating compositions, and applying and thermally curinga clear coating composition.

The clear coating composition used for the above method is usually anorganic solvent-based coating composition comprising ahydroxyl-containing acrylic resin and a melamine resin, a powder coatingcomposition, an organic solvent-based coating composition to be cured bycrosslinking between a carboxyl group and an epoxy group.

However, it was found that when the 3-coat 2-bake method employs, as theclear coating composition, a powder coating composition or an organicsolvent-based coating composition to be cured by crosslinking between acarboxyl group and an epoxy group, the method has the drawback that, ifthe coating surface of the second coating composition is not sandedbefore application of the clear coating composition, the intercoatadhesion between the second coating composition and the clear coatingcomposition becomes insufficient.

DISCLOSURE OF THE INVENTION

An object of the present invention is to provide a novel method offorming a multilayered topcoat film free from the above drawback of theprior art.

Another object of the present invention is to provide a novel method offorming a multilayered topcoat film excellent in intercoat adhesion,finish properties such as film appearance and gloss, and filmperformance properties such as solvent resistance, weather resistanceand water resistance.

Other objects and features of the present invention will be apparentfrom the following description.

The present invention provides a 3-coat 2-bake method of forming amultilayered topcoat film, comprising applying a first coatingcomposition (A) and a second coating composition (B) to a substrate,thermally curing the two compositions, and applying and thermally curinga clear coating composition (C);

the first coating composition (A) being an organic solvent-based coloredcoating composition;

the second coating composition (B) being an organic solvent-basedcoating composition comprising an acrylic resin (b-1) whose main chainhas, bonded thereto, at least two side chains of different lengths eachhaving at least one hydroxyl group, a polyepoxide (b-2) and acrosslinking agent (b-3); and

the clear coating composition (C) being a powder coating composition(C-1) or an organic solvent-based coating composition (C-2) comprising ahydroxyl- and carboxyl-containing resin (c-2a) and a polyepoxide (c-2b).

The present inventors conducted extensive research to develop a 3-coat2-bake method of forming a multilayered topcoat film having improvedintercoat adhesion between the second coating composition and the clearcoating composition (which is a powder coating composition or an organicsolvent-based coating composition to be cured by crosslinking between acarboxyl group and an epoxy group), without impairing finish propertiesand film performance of the multilayered topcoat film. As a result, theyfound that when the above specified organic solvent-based coatingcomposition is used as the second coating composition, the intercoatadhesion between the second coating composition and the clear coatingcomposition can be improved, without impairing the excellent finishproperties and high film performance of the multilayered topcoat filmand without necessitating sanding of the coating surface of the secondcoating composition.

The present invention has been accomplished based on these findings.

The method of forming a multilayered topcoat film of the presentinvention will be described below in further detail.

Substrate

Substrates usable in the method of the invention include metallic orplastic materials for automobiles, electric appliances, etc.; thesematerials as coated with an undercoat such as a cationicelectrodeposition coat; and these materials as coated with the undercoatand an intermediate coat.

First Coating Composition (A)

In the method of the invention, the first coating composition (A) to beapplied to the substrate is an organic solvent-based colored coatingcomposition.

Usable as the composition (A) are known thermosetting coatingcompositions comprising a base resin, a crosslinking agent, a coloringpigment and an organic solvent.

Examples of base resins include acrylic resins, vinyl resins, polyesterresins, alkyl resins and urethane resins, each having hydroxyl, epoxy,carboxyl, alkoxysilane or like crosslinkable functional group. Thesebase resins may be used either singly or in combination.

Examples of crosslinking agents include alkyl-etherified melamineresins, urea resins, guanamine resins, polyisocyanate compounds, blockedpolyisocyanate compounds, epoxy compounds and carboxyl-containingcompounds. These crosslinking agents may be used either singly or incombination.

The proportions of the base resin and the crosslinking agent are usuallyabout 50 to 90 wt. % of the base resin and about 50 to 10 wt. % of thecrosslinking agent, based on the total amount of the two components.

Usable coloring pigments include titanium oxide, zinc oxide, carbonblack, cadmium red, molybdenum red, chrome yellow, chrome oxide,Prussian blue, cobalt blue and like inorganic solid coloring pigments;azo pigments, phthalocyanine pigments, quinacridone pigments,isoindorine pigments, vat pigments, perylene pigments and like organicsolid coloring pigments; flaky aluminum and like metallic pigments; andmica, metal oxide-coated mica, micaceous iron oxide and likelight-interference coloring pigments. These pigments may be used eithersingly or in combination.

These coloring pigments are suitably selected to obtain a solid colorcoating composition, a metallic coating composition, alight-interference color coating composition or a coating compositionhaving a combination of color characteristics of these coatingcompositions. The amount of the coloring pigment to be added can bedetermined in consideration of the hiding power described hereinafter.

Usable organic solvents include ordinary solvents for use in coatingcompositions, such as hexane, heptane, xylene, toluene, cyclohexane andlike hydrocarbon solvents; methyl acetate, ethyl acetate, ethyleneglycol monomethyl ether acetate, diethylene glycol monomethyl etheracetate and like ester solvents; isopropyl ether, ethylene glycolmonomethyl ether, diethylene glycol monobutyl ether and like ethersolvents; ethyl alcohol, butyl alcohol, hexyl alcohol and like alcoholsolvents; and methyl isobutyl ketone, methyl ethyl ketone, isophorone,acetophenone and like ketone solvents.

The first coating composition (A) may further contain, where necessary,an extender pigment, a UV absorber, a photostabilizer, a flow modifier,an anti-cissing agent or like ordinary coating additive.

It is preferable that the composition (A) has a solid content of about20 to 70 wt. %, preferably about 30 to 70 wt. %, at the time ofapplication. The viscosity of the composition (A) is adjusted preferablyto about 10 to 60 second (Ford Cup #4/20° C.) for application.

The single-layer coating of the composition (A) preferably has a hidingpower sufficient to mask the color of the coating surface below thecoating. Specifically stated, the composition (A) has such a hidingpower that a 15 μm thick cured coating of the composition (A) has alight transmittance of 3% or less in the wavelength range of 400 to 700nm.

The composition (A) can be applied by airless spraying, air spraying,electrostatic coating or like coating technique, to such a thickness asto form a cured coating of about 10 to 50 μm. The applied composition isallowed to stand, where necessary, for several minutes at roomtemperature to about 100° C., and then the second coating composition(B) described below is applied to the uncured coating surface of thecomposition (A).

Coating Composition (B)

In the method of the invention, the second coating composition (B) to beapplied to the uncured coating surface of the first coating composition(A) is an organic solvent-based thermosetting coating compositioncomprising an acrylic resin (b-1) whose main chain has, bonded thereto,at least two side chains of different lengths each having at least onehydroxyl group, and a polyepoxide (b-2) and a crosslinking agent (b-3).

It is preferable that the composition (B) forms a colorless clearcoating or a clear coating colored in such an extent that the color ofthe coating of the composition (A) can be seen through the coating ofthe composition (B). Stated specifically, the composition (B) preferablyhas such a transparency that a 15 μm thick cured coating of thecomposition (B) has a light transmittance of about 30 to 100% in thewavelength range of 400 to 700 nm. Because of the transparency of thecomposition (B), the multilayered topcoat film formed by the method ofthe invention has an appearance of depth and excellent decorativeproperties.

The acrylic resin (b-1) is made of a polymer chain containing an acrylicpolymerizable monomer as an essential constituent, the polymer chaincomprising a main chain and at least two side chains each having atleast one hydroxyl group. The at least two side chains are different inlength from each other, and bonded in a pendant-like fashion to the mainchain. The hydroxyl group of each side chain is linked to the main chainof the acrylic resin via an atomic chain comprising carbon atom(s),oxygen atom(s), nitrogen atom(s) or other atom(s). Specifically, theatomic chain may be, for example, a divalent hydrocarbon group, or adivalent hydrocarbon group containing an ester bond, an ether bond, anamide bond or like bond. The length of the side chain means the numberof atoms of the atomic chain linking the hydroxyl group to the mainchain. It is essential for the acrylic resin (b-1) to have at least twoside chains of different lengths.

Of the at least two side chains each bonded to the main chain of theacrylic resin (b-1) and having at least one hydroxyl group, a side chainhaving a longer atomic chain between the hydroxyl group and the mainchain (hereinafter sometimes referred to as “longer sidechain-hydroxyl”) is different in length from a side chain having ashorter atomic chain between the hydroxyl group and the main chain(hereinafter sometimes referred to as “shorter side chain-hydroxyl”) byat least 1, preferably at least 2, more preferably 2 to 20, in terms ofthe number of atoms constituting the side chains.

The acrylic resin (b-1) is obtainable by, for example, copolymerizinghydroxyl-containing polymerizable monomers and an acrylic polymerizablemonomer as essential components and, where necessary, anotherpolymerizable monomer.

Hydroxyl-containing polymerizable monomers are compounds each having atleast one hydroxyl group and at least one polymerizable unsaturated bondper molecule, such as hydroxyethyl (meth)acrylate, hydroxypropyl(meth)acrylate, hydroxybutyl (meth)acrylate and like C₂ to C₈hydroxyalkyl esters of (meth)acrylic acids; monoester of (meth)acrylicacids and polyether glycols such as polyethylene glycol, polypropyleneglycol and polybutylene glycol; monethers of the above hydroxyalkylesters and polyether glycols such as polyethylene glycol, polypropyleneglycol and polybutylene glycol; adducts of α, β-unsaturated carboxylicacids and monoepoxy compounds such as Cardula E-10 (tradename, a productof Shell Chemical Co.) and α-olefin epoxides; adducts of glycidyl(meth)acrylates and monobasic acids such as acetic acid, propionic acidand p-t-butylbenzoic acid; lactone-modified acrylic monomers obtainableby reacting one of the above hydroxyalkyl esters with 1 to 5 moles of alactone such as ε-caprolactone, β-methyl-δ-valerolactone,γ-valerolactone, δ-valerolactone, δ-caprolactone, γ-caprolactone,β-propiolactone or γ-butyrolactone; and N-methylol (meth)acrylamide andlike amide monomers.

Commercial products of the lactone-modified acrylic monomers include,for example, Placcel FA-1, Placcel FA-2 and Placcel FA-3 (monomersprepared by addition reaction of hydroxyethyl acrylate andε-caprolactone), Placcel FM-1, Placcel FM-3 and Placcel FM-5 (monomersrespectively prepared by addition reaction of 1 mole of hydroxyethylmethacylate and 1, 3 or 5 moles of ε-caprolactone), all available fromDaicel Chemical Industries Co., Ltd.; and TONEm-100 (a monomer preparedby addition reaction of 1 mole of hydroxyethyl acrylate and 2 moles ofε-caprolactone).

The hydroxyl-containing acrylic resin for use in the invention can beobtained also by reacting a hydroxyl-containing acrylic resin and any ofthe above lactones.

An acrylic resin having a longer side chain-hydroxyl and a shorter sidechain-hydroxyl can be obtained by using, as hydroxyl-containingpolymerizable monomers, at least two monomers different from each otherin the length of the atomic chain intervening between the hydroxyl groupand the polymerizable unsaturated bond.

Examples of acrylic polymerizable monomers include monoesters of(meth)acrylic acids and C₁ to C₂₄ monovalent aliphatic or alicyclicalcohols, such as methyl (meth)acrylate, ethyl (meth)acrylate, propyl(meth)acrylate, butyl (meth)acrylate, hexyl (meth)acrylate, 2-ethylhexyl(meth)acrylate, octyl (meth)acrylate, decyl (meth)acrylate, lauryl(meth)acrylate and cyclohexyl (meth)acrylate.

The other polymerizable monomer is a polymerizable unsaturatedgroup-containing compound other than the above hydroxyl-containingpolymerizable monomers and acrylic polymerizable monomers. Usable as theother monomer are, for example, the following:

i) carboxyl-containing polymerizable monomers such as acrylic acid,methacrylic acid, crotonic acid, itaconic acid, maleic acid and fumaricacid;

ii) polymerizable amide monomers such as N-methoxymethyl(meth)acrylamide and N-butoxymethyl (meth)acrylamide;

iii) vinyl ethers such as ethyl vinyl ether, propyl vinyl ether, butylvinyl ether, hexyl vinyl ether, cyclopentyl vinyl ether, cyclohexylvinyl ether, phenyl vinyl ether, benzyl vinyl ether and allyl glycidylether;

iv) vinyl acetate, vinyl propionate, ethylene, propylene, vinylchloride, styrene, α-methylstyrene, N,N-dimethylaminoethyl(meth)acrylate, N,N-diethylaminoethyl (meth)acrylate, (meth)acrylamide,(meth)acrylonitrile, vinyl pyrrolidone and the like;

v) epoxy-containing polymerizable monomers such as glycidyl(meth)acrylate, methyl glycidyl (meth)acrylate, allyl glycidyl ether and3,4-epoxycyclohexyl methyl (meth)acrylate.

The acrylic resin (b-1) can be prepared by carrying out, in an ordinarymanner, solution polymerization of the hydroxyl-containing polymerizablemonomers and acrylic polymerizable monomer as essential components and,where necessary the other polymerizable monomer, using, for example, aradical polymerization catalyst.

The proportions of these monomers can be selected so that the resultingacrylic resin has a hydroxyl value of 50 to 200 mg KOH/g, preferably 70to 150 mg KOH/g. It is desirable that the acrylic resin has a numberaverage molecular weight of 3,000 to 50,000, in particular 5,000 to20,000. The hydroxyl value specified above is the total of the hydroxylvalue of the longer side chain-hydroxyl and the hydroxyl value of theshorter side chain-hydroxyl. It is preferable that the proportions ofthe hydroxyl value of the longer side chain-hydroxyl and the hydroxylvalue of the shorter side chain-hydroxyl are 10 to 90%, in particular 30to 50%, of the former, and 90 to 10%, in particular 70 to 50%, of thelatter.

The polyepoxide (b-2) is a compound having an average of at least 2epoxy groups per molecule. The polyepoxide preferably has a numberaverage molecular weight of about 120 to 200,000, in particular about240 to 80,000. Polyepoxides having a number average molecular weightless than about 120 are difficult to obtain, while those having a numberaverage molecular weight more than about 200,000 are difficult to handlebecause of their increased viscosity, and result in a coating poor insolvent resistance, scratch resistance and other properties owing totheir large molecular weight between crosslinks.

Specific examples of polyepoxides usable as the polyepoxide (b-2)include polymers obtainable by radical polymerization of anepoxy-containing ethylenically unsaturated monomer [e.g., glycidyl(meth)acrylate, methyl glycidyl (meth)acrylate, allyl glycidyl ether orlike glycidyl-containing ethylenically unsaturated monomer,3,4-epoxycyclohexylmethyl (meth)acrylate or like alicyclicepoxy-containing ethylenically unsaturated monomer] and, wherenecessary, another vinyl monomer such as a hydroxyl-containingethylenically unsaturated monomer (e.g., the above hydroxyl-containingacrylic monomers), any of the above C₁ to C₂₄ alkyl (meth)acrylates,cycloalkyl (meth)acrylate or an aromatic vinyl monomer; glycidyl ethercompounds such as diglycidyl ether, 2-glycidyl phenyl glycidyl ether and2,6-diglycidyl phenyl glycidyl ether; glycidyl- and alicyclicepoxy-containing compounds such as vinylcyclohexene dioxide and limonenedioxide; and alicyclic epoxy-containing compounds such asdicyclopentadiene dioxide, bis(2,3-epoxycyclopentyl)ether,epoxycyclohexene carboxylic acid ethylene glycol diester,bis(3,4-epoxycyclohexylmethyl)adipate,3,4-epoxycyclohexylmethyl-3,4-epoxycyclohexane carboxylate, and3,4-epoxy-6-methylcyclohexylmethyl-3,4-epoxy-6-methylcyclohexanecarboxylate. Of these examples, preferred are polymers containing, asmonomer components, an epoxy-containing ethylenically unsaturatedmonomer and optionally a hydroxyl-containing ethylenically unsaturatedmonomer, since these polymers have good low-temperature curability.

Usable as the crosslinking agent (b-3) are, for example,alkyl-etherified melamine resins, urea resins, guanamine resins,polyisocyanate compounds and blocked polyisocyanate compounds.

The second coating composition (B) is an organic solvent-based coatingcomposition comprising an acrylic resin (b-1), a polyepoxide (b-2) and acrosslinking agent (b-3). Usable organic solvents include those shownhereinabove for use in the composition (A).

The proportions of the above components of the composition (B) are notlimited and may be varied according to the kinds of the components orother factors. It is usually suitable to use, relative to 100 parts byweight of the acrylic resin (b-1), about 5 to 150 parts by weight of thepolyepoxide (b-2) and about 5 to 150 parts by weight of the crosslinkingagent (b-3).

From the viewpoint of improvement in scratch resistance of themultilayered film, it is preferable that the composition (B) contains analkoxysilane-containing acrylic resin, in addition to the acrylic resin(b-1), the polyepoxide (b-2) and the crosslinking agent (b-3).

The alkoxysilane-containing acrylic resin is obtainable bycopolymerizing an alkoxysilane-containing polymerizable monomer and anacrylic polymerizable monomer as essential components and, wherenecessary, a hydroxyl-containing polymerizable monomer and/or anotherpolymerizable monomer.

The alkoxysilane-containing polymerizable monomer is a compound havingat least one alkoxysilane group and at least one polymerizableunsaturated bond per molecule. Examples of such compounds include vinyltrimethoxysilane, vinyl methyl dimethoxysilane, vinyl triethoxysilane,vinyl methyl diethoxysilane, vinyl tris(2-methoxyethoxy)silane,γ-(meth)acryloyloxypropyl-trimethoxysilane,γ-(meth)acryloyloxypropylmethyl-dimethoxysilane, vinyl triacetoxysilane,β-(meth)acryloyloxyethyltrimethoxysilane,γ-(meth)acryloyloxypropyltriethoxysilane, andγ-(meth)acryloyloxypropylmethyldiethoxysilane.

The acrylic polymerizable monomer and hydroxyl-containing polymerizablemonomer may be any of those shown as the monomers for forming theacrylic resin (b-1). Further, it is suitable to select the otherpolymerizable monomer from monomers shown as the other polymerizablemonomer constituting the acrylic resin (b-1).

The composition (B) may further contain, where necessary, a curingcatalyst, a coloring pigment, an extender pigment, a UV absorber, aphotostabilizer, a flow modifier, an anti-cissing agent or like ordinarycoating additive.

In particular, a coloring pigment may or may not be added to thecomposition (B) so as to obtain a coating composition for forming acolorless clear coating or a coating composition for forming a clearcoating colored in such an extent that the color of the coating of thecomposition (A) can be seen through the coating of the composition(B).Any of the coloring pigments shown hereinbefore as pigments usable forthe composition (A) can be employed for the composition (B). One or morecoloring pigments may be selected to prepare a solid color coatingcomposition, a metallic coating composition, a light-interference colorcoating composition or a coating composition having a combination ofcolor characteristics of these coating compositions. The amount of thecoloring pigment to be added can be determined in consideration of theabove specified transparency of the coating of the composition (B).

Usable curing catalysts include, for example, dibutyltin dilaurate andlike tin compounds, aluminum chelate compounds, titanium chelatecompounds, zirconium chelate compounds, aluminum ester, tetramethyltitanate, tetrapentyl titanate and like titanates, and tetramethylzirconate, tetrapentyl zirconate and like zirconates. The curingcatalyst, when employed, is used preferably in a proportion of about 0.1to 10 parts by weight relative to 100 parts by weight of the totalamount of the acrylic resin (b-1), the polyepoxide (b-2) and thecrosslinking agent (b-3).

It is suitable that the composition (B) has a solid content of about 20to 70 wt. %, preferably about 30 to 70 wt. %, at the time ofapplication. Further, the viscosity of the composition (B) is adjustedpreferably to about 10 to 60 seconds (Ford cup #4/20° C.) forapplication.

The composition (B) is applied to the uncured coating surface of thecomposition (A) by airless spraying, air spraying, electrostatic coatingor like coating technique, to such a thickness as to form a curedcoating of about 10 to 50 μm. The two compositions are cured andcrosslinked by heating at about 100 to 180° C., preferably about 120 to160° C., for about 10 to 40 minutes. Then, the clear coating composition(C) described below is applied to the cured coating surface.

When the composition (B) is a coating composition that forms a coloredclear coating having the specific transparency, the multilayer coatingformed by applying the composition (B) to the colored coating surface ofthe composition (A) has improved decorative and aesthetic properties ascompared with the single-layer coating of the composition (A), due tothe combination of the colors of the two coatings, such as solid color,metallic color, light-interference color, etc.

Clear Coating Composition (C)

The clear coating composition (C) is a composition to be applied to thecured coating surface of the composition (B), for forming a clearcoating. The composition (C) is a powder coating composition (C-1) or anorganic solvent-based thermosetting coating composition (C-2) comprisinga hydroxyl- and carboxyl-containing resin (c-2a) and a polyepoxide(c-2b).

The powder coating composition (C-1) comprises a base resin and acrosslinking agent as main components.

Usable base resins include, but are not limited to, acrylic resins,polyester resins, fluorocarbon resins and urethane resins, each havingat least one crosslinkable functional group selected from amonghydroxyl, carboxyl, glycidyl and like group; and graft polymers or likemodification products of these resins. Particularly preferred baseresins include glycidyl-containing acrylic resins.

It is generally preferable that the base resin has a glass transitiontemperature of about 50° C. or higher, in particular 60 to 120° C. Themolecular weight of the base resin is not limited and can be suitablyselected according to the purpose.

The crosslinking agent is used to three-dimensionally crosslink and curethe base resin when heated. Examples of crosslinking agents includealkyl-etherified melamine resins, blocked polyisocyanate compounds,epoxy compounds, isocyanurate compounds and aliphatic dibasic acids,among which aliphatic dibasic acids are particularly preferred.

The base resin and the crosslinking agent are used, most preferably, insuch proportions that the molar ratio of the functional group in thebase resin to the functional group in the crosslinking agent is about1:1.

The powder coating composition (C-1) may further contain, wherenecessary, a flow modifier, a UV absorber, a photostabilizer or likecoating additive.

The powder coating composition (C-1) can be prepared by melting andkneading the above components and cooling the melt, followed by grindingto a suitable particle size.

The composition (C-1) may be applied by any powder coating technique,such as electrostatic spraying or fluidized-dipping, without limitation.

The thickness of the coating of the powder coating composition is notlimited, but a thickness of 20 to 200 μm (when cured) is usuallysuitable. A thickness of 20 to 120 μm is particularly preferred toobtain a coating that is good in smoothness, image sharpness, gloss andapparent thickness. The powder coating composition is cured usually at,for example, about 120 to 180° C.

The organic solvent-based coating composition (C-2) is a thermosettingcoating composition comprising a hydroxyl- and carboxyl-containing resin(c-2a), a polyepoxide (c-2b) and an organic solvent as essentialcomponents.

The resin (c-2a) contains carboxyl groups in an amount of about 15 to150 mg KOH/g, preferably about 20 to 100 mg KOH/g, calculated as an acidvalue. If the acid value is less than about 15 mg KOH/g, thelow-temperature curability and other properties are impaired, whereas ifthe acid value is more than about 150 mg KOH/g, the resulting coatingwill be poor in water resistance, weather resistance, etc. Thus, acidvalues outside the specified range are undesirable.

The resin (c-2a) contains hydroxyl groups in an amount of about 20 to300 mg KOH/g, preferably about 20 to 200 mg KOH/g, calculated as ahydroxyl value. If the hydroxyl value is less than about 20 mg KOH/g,the low-temperature curability, durability and other properties of thecoating will be impaired, whereas if the hydroxyl value is more thanabout 300 mg KOH/g, the water resistance and other properties of theresulting coating will be impaired because of a large amount ofunreacted hydroxyl groups remaining in the coating. Thus, hydroxylvalues outside the specified range are undesirable.

It is preferable that the resin (c-2a) has a number average molecularweight of about 3,000 to 200,000, in particular about 5,000 to 80,000.If the number average molecular weight is less than about 3,000, theresulting coating will have lowered durability, whereas if the numberaverage molecular weight is more than about 200,000, the resin isdifficult to handle because of an increased viscosity. Thus, numberaverage molecular weights outside the specified range are undesirable.

The resin (c-2a) is not limited in kind and may be suitably selectedfrom known resins. Preferred are, for example, acrylic resins,fluorocarbon resins and polyester resins, from the viewpoints of weatherresistance, durability and other properties of the coating.

Specific examples of acrylic resins include those prepared bycopolymerizing a hydroxy-containing acrylic monomer and acarboxyl-containing ethylenically unsaturated monomer as essentialmonomer components, and where necessary, another ethylenicallyunsaturated monomer. Examples of hydroxyl-containing acrylic monomersinclude hydroxyethyl (meth)acrylate, hydroxypropyl (meth)acrylate,hydroxybutyl (meth)acrylate, (poly)ethylene glycol mono(meth)acrylate,(poly)propylene glycol mono(meth)acrylate, and adducts of these monomersand caprolactones such as ε-caprolactone. Examples ofcarboxyl-containing ethylenically unsaturated monomers include(meth)acrylic acid, maleic acid and maleic anhydride. Examples of otherethylenically unsaturated monomers include methyl (meth)acrylate, ethyl(meth)acrylate, propyl (meth)acrylate, butyl (meth)acrylate, octyl(meth)acrylate, 2-ethylhexyl (meth)acrylate, stearyl (meth)acrylate,cyclohexyl (meth)acrylate and like C₁ to C₂₄ alkyl (meth)acrylates orcycloalkyl (meth)acrylates; styrene, vinyl toluene and like aromaticvinyl monomers; (meth)acrylonitrile and like nitrile compounds; and(meth)acrylamide, N-methylol (meth)acrylamide and like amide compounds.

Specific examples of acrylic resins other than those mentioned aboveinclude a half-esterified product of a hydroxyl-containing acrylic resinobtainable by copolymerizing any of the above hydroxyl-containingacrylic monomers and, where necessary, another ethylenically unsaturatedmonomer, the half-esterified product being produced by reacting part ofhydroxyl groups of the acrylic resin with a polybasic anhydride. Usablepolybasic anhydrides include maleic anhydride, itaconic anhydride,succinic anhydride, HET anhydride, himic anhydride, phthalic anhydride,1,2-cyclohexanedicarboxylic anhydride, hexahydrophthalic anhydride,trimellitic anhydride and pyromellitic anhydride.

Specific examples of fluorocarbon resins include resins having afluorine group in their side chain and resins having a carboxyl groupintroduced thereto. The fluorocarbon resins having a fluorine group intheir side chain are obtainable by, for example, copolymerizing any ofthe above hydroxyl-containing acrylic monomers, any of the abovecarboxyl-containing ethylenically unsaturated monomers, and ameth(acrylate) monomer containing a perfluoroalkyl or perfluoroalkenylgroup [e.g., perfluorobutylethyl (meth)acrylate, perfluorooctylethyl(meth)acrylate, perfluoroisononylethyl (meth)acrylate orperflurodecylethyl (meth)acrylate and, where necessary, any of the otherethylenically unsaturated monomers mentioned above. The resins having acarboxyl group introduced thereto can be prepared by, for example,reacting any of the above polybasic anhydrides with part of hydroxylgroups of a hydroxyl-containing resin obtainable by copolymerizing ahydroxyl-containing vinyl monomer (e.g., hydroxyethyl vinyl ether,hydroxypropyl vinyl ether, hydroxybutyl vinyl ether or (poly)ethyleneglycol monoallyl ether), a fluoroolefin (e.g., vinyl fluoride,vinylidene fluoride, ethylene trifluoride chloride or ethylenetetrafluoride) and, where necessary, another ethyleneically unsaturatedmonomer (e.g., ethylene, propylene, butylene, ethyl vinyl ether, butylvinyl ether, cyclohexyl vinyl ether, vinyl acetate, vinyl butylate orvinyl propionate). The hydroxyl-containing resin for use in preparationof the resins having a carboxyl group introduced thereto may be oneprepared using, as an essential monomer component, a carboxyl-containingethylenically unsaturated monomer such as (meth)acrylic acid, maleicacid, fumaric acid, itaconic acid or citraconic acid.

The polyester resins mentioned above are chiefly esters of polybasicacids and polyhydric alcohols.

A mainly used polybasic acid is at least one dibasic acid selected fromamong phthalic anhydride, isophthalic acid, terephthalic acid, succinicacid, adipic acid, fumaric acid, maleic anhydride, tetrahydrophthalicanhydride, hexahydrophthalic anhydride and the like. Also usable wherenecessary are benzoic acid, crotonic acid, p-tert-butylbenzoic acid orlike monobasic acid, trimellitic anhydride, methylcyclohexenetricarboxylic acid, pyromellitic anhydride and like tri- or higherpolybasic acid, and so on.

A mainly used polyhydric alcohol is, for example, ethylene glycol,propylene glycol, diethylene glycol, butanediol, neopentyl glycol,cyclohexane dimethanol, 1,6-hexanediol or like dihydric alcohol. Wherenecessary, the dihydric alcohol is used in combination with glycerine,trimethylolethane, trimethylolpropane, pentaerythritol or like tri- orhigher polyhydric alcohol.

The polyepoxide (c-2b) in the organic solvent-based coating composition(C-2) for use in the present invention is a compound having an averageof at least about 2 epoxy groups per molecule. The polyepoxide (c-2b)serves as a crosslinking agent for the resin (c-2a).

The polyepoxide (c-2b) may be the same as the polyepoxide (b-2) in thesecond coating composition (B), and any of polyepoxides shown as thepolyepoxide (b-2) can be employed as the polyepoxide (c-2b).

In the organic solvent-based thermosetting coating composition (C-2),the hydroxyl- and carboxyl-containing resin (c-2a) and the polyepoxide(c-2b) are used, based on the total amount of the two components, in thefollowing proportions:

The resin (C-2a) is used in a proportion of about 5 to 95 wt. %,preferably about 15 to 85 wt. %. If the proportion is less than about 5wt. %, the base resin content of the composition will be too small, thusresulting in a coating poor in finished appearance and performance. Onthe other hand, if the proportion is more than about 95 wt. %, thecurability of the composition will be impaired. Thus, proportionsoutside the specified range are undesirable.

The polyepoxide (C-2b) is used in a proportion of about 5 to 95 wt. %,preferably about 15 to 85 wt. %. If the proportion of the polyepoxide(C-2b) is outside the specified range, the curability of the compositionwill reduce, hence undesirable.

The organic solvent in the coating composition (C-2) is a component fordissolving or dispersing the components (c-2a) and (c-2b). The organicsolvent is not limited as long as it is capable of dissolving ordispersing the components (c-2a) and (c-2b), and can be suitablyselected from known organic solvents, including those usable for thefirst coating composition (A).

The organic solvent is added in such an amount that the coatingcomposition (C-2) has a solid content of about 10 to 70 wt. %. It issuitable that the viscosity of the coating composition (C-2) at the timeof application is adjusted to about 10 to 60 seconds (Ford Cup #4/20°C.).

The coating composition (C-2) may contain, where necessary, a curingcatalyst, an extender pigment, a UV absorber, a UV stabilizer, a flowmodifier or like coating additive, in addition to the hydroxyl- andcarboxyl-containing resin (c-2a), the polyepoxide (c-2b) and the organicsolvent.

The coating technique for applying the organic solvent-basedthermosetting coating composition (C-2) is not limited, and may be airspraying, airless spraying, electrostatic air spraying, electrostaticairless spraying, rotary atomization or like technique.

Coating Procedure

The method of forming a multilayered topcoat film of the presentinvention is a 3-coat 2-bake coating method comprising applying to asubstrate the first coating composition (A) and second coatingcomposition (B) described above, thermally curing the two compositions,and applying and thermally curing the clear coating composition (C).

More specifically, the coating procedure is as follows: The firstcoating composition (A) of a solid, metallic or light-interference coloris applied to a substrate by airless spraying, air spraying,electrostatic coating or like coating technique, to such a thickness asto form a cured coating having a thickness of about 10 to 50 μm, andwhere necessary, the coated substrate is allowed to stand for severalminutes at room temperature. Then, the second coating composition (B) isapplied.

The second coating composition (B) may be a composition containing nocoloring pigment, or a composition containing a coloring pigment in suchan amount that the composition (B) forms a clear coating of a solid,metallic or light-interference color, through which the color of thecoating of the composition (A) can be seen through the coating of thecomposition (B). The composition (B) is applied to the uncured coatingsurface of the composition (A) by airless spraying, air spraying,electrostatic coating or like technique, to such a thickness as to forma cured coating having a thickness of about 10 to 50 μm. Wherenecessary, after being allowed to stand at room temperature, thecompositions (A) and (B) are at the same time cured and crosslinked byheating at about 100 to 180° C., preferably about 120 to 160° C., forabout 10 to 40 minutes.

Subsequently, the clear coating composition (C) is applied to the curedcoating surface of the composition (B), by airless spraying, airspraying, electrostatic coating or like technique, to such a thicknessas to form a cured coating having a thickness of about 20 to 200 μm, andcured and crosslinked by heating at about 100 to 180° C., preferablyabout 120 to 160° C., for about 10 to 40 minutes, thereby giving amultilayered topcoat film.

It is preferable that the viscosity of each of the first coatingcomposition (A), the second coating composition (B) and the clearcoating composition (C) at the time of application is adjusted to about10 to 60 seconds (Ford Cup #4/20° C.).

The present invention provides a 3-coat 2-bake method of forming amultilayered topcoat film, comprising applying a first coatingcomposition and a second coating composition to a substrate, thermallycuring the two compositions, applying as a clear coating composition, apowder coating composition or an organic solvent-based coatingcomposition to be cured by crosslinking of a carboxyl group and an epoxygroup, and thermally curing the clear coating composition; in whichmethod the second coating composition and the clear coating compositionhave remarkably improved intercoat adhesion. Further, the obtainedmultilayered topcoat film is excellent in finish properties such asappearance and gloss, and performance properties such as solventresistance, weather resistance and water resistance.

Accordingly, the method of the present invention is extremely useful asa method for forming a topcoat film on an automobile body or the like.

BEST MODE FOR CARRYING OUT THE INVENTION

The following Production Examples, Examples and Comparative Examples areprovided to illustrate the present invention in further detail. In theseexamples, parts and percentages are all by weight.

PRODUCTION EXAMPLE 1 Production of First Coating Composition (A)

A coating composition (A-1) was obtained by mixing together 65 parts (assolids, the same applies hereinafter) of a polyester resin (hydroxylvalue: 150 mg KOH/g, acid value: 5 mg KOH/g, number average molecularweight: 3,000), 35 parts of a melamine resin (tradename “U-Van 28-60”, aproduct of Mitsui Totatsu Chemicals, Inc.) and 10 parts of carbon black,followed by dilution with xylene to a viscosity of 14 seconds (Ford Cup#4/20° C.). The composition (A-1) had such a hiding power that a 15 μmthick cured coating of the composition (A-1) had a light transmittanceof 0.1% or less in the light wavelength range of 400 to 700 nm.

PRODUCTION EXAMPLE 2 Production of Hydroxyl-containing Acrylic Resin(b-1a)

A hydroxyl-containing acrylic resin (b-1a) was obtained bycopolymerizing in an ordinary manner 37 parts of Placcel FA-2(tradename, a product of Daicel Chemical Industries Co., Ltd., a monomerprepared by addition reaction of hydroxyethyl acrylate andE-caprolactone), 23 parts of hydroxybutyl acrylate, 2 parts of acrylicacid, 18 parts of n-butyl acrylate and 20 parts of styrene. The resin(b-1a) had a hydroxyl value of 150 mg KOH/g, an acid value of 15 mgKOH/g and a number average molecular weight of 10,000. Of the hydroxylvalue, 40% was attributable to Placcel FA-2, and 60% to hydroxybutylacrylate. In the resin (b-1a), the difference in length between thelonger side chain-hydroxyl and the shorter side chain-hydroxyl was 12atoms.

PRODUCTION EXAMPLE 3 Production of Hydroxyl-containing Acrylic Resin(b-1b)

A hydroxyl-containing arylic resin (b-1b) was obtained by copolymerizing12.5 parts of hydroxyethyl acrylate, 23 parts of hydroxybutyl acrylate,2 parts of acrylic acid, 42.5 parts of n-butyl acrylate and 20 parts ofstyrene. The resin (b-1b) had a hydroxyl value of 150 mg KOH/g, an acidvalue of 15 mg KOH/g and a number average molecular weight of 10,000. Ofthe hydroxyl value, 40% was attributable to hydroxyethyl acrylate, and60% to hydroxybutyl acrylate. In the resin (b-1b), the difference inlength between the longer chain-hydroxyl and the shorter sidechain-hydroxyl was 2 atoms.

PRODUCTION EXAMPLE 4 Production of Comparative Hydroxyl-containingAcrylic Resin (b-1c)

A hydroxyl-containing acrylic resin (b-1c) was obtained bycopolymerizing in an ordinary manner 12.5 parts of hydroxyethylacrylate, 21 parts of hydroxyethyl methacrylate, 2 parts of acrylicacid, 44.5 parts of n-butyl acrylate and 20 parts of styrene. The resin(b-1c) had a hydroxyl value of 150 mg KOH/g, an acid value of 15 mgKOH/g and a number average molecular weight of 10,000. Of the hydroxylvalue, 40% was attributable to hydroxyethyl acrylate, and 60% tohydroxyethyl methacrylate. In the resin (b-1c), the difference in sidechain length between the longer side chain-hydroxyl and the shorter sidechain-hydroxyl is 0.

PRODUCTION EXAMPLE 5 Production of Comparative Hydroxyl-containingAcrylic Resin (b-1d)

A hydroxyl-containing acrylic resin (b-1d) was obtained bycopolymerizing in an ordinary manner 31 parts of hydroxyethyl acrylate,2 parts of acrylic acid, 47 parts of n-butyl acrylate and 20 parts ofstyrene. The resin (b-1d) had a hydroxyl value of 150 mg KOH/g, an acidvalue of 15 mg KOH/g and a number average molecular weight of 10,000.

PRODUCTION EXAMPLE 6 Production of Polyepoxide (b-2a)

A polyepoxide (b-2a), i.e., a glycidyl- and hydroxyl-containing acrylicresin, was obtained by copolymerizing in an ordinary manner 40 parts ofglycidyl methacrylate, 21 parts of hydroxyethyl acrylate and 39 parts ofn-butyl acrylate. The resin (b-2a) had an epoxy equivalent of 355, ahydroxyl value of 100 mg KOH/g and a number average molecular weight of10,000.

PRODUCTION EXAMPLE 7 Production of polyepoxide (b-2b)

The polyepoxide (b-2b) was a 3,4-epoxycyclohexylmethyl-3,4-cyclohexanecarboxylate (tradename “ERL-4221”, a product of Union Carbide Corp.)having an epoxy equivalent of 138, an acid value of 0 mg KOH/g and anumber average molecular weight of 276.

PRODUCTION EXAMPLE 8 Production of Second Coating Composition (B)

Second coating compositions (B)-(1) to (B)-(7) were obtained by mixingthe components shown in Table 1 in the proportions (as solids) shown inthe table, followed by dilution with “Swasol 1000” (tradename, ahydrocarbon solvent manufactured by Cosmo Oil Co., Ltd.) to a viscosityof 20 second (Ford Cup #4/20° C.). Table 1 shows the light transmittance(%) of 15 μm thick cured coatings of the second coating compositions(B)-(1) to (B)-(7) in the wavelength range of 400 to 700 nm.

TABLE 1 Second coating composition (B) (1) (2) (3) (4) (5) (6) (7)Hydroxyl- containing acrylic resin b-1a 100 100 100 100 b-1b 100 b-1c100 b-1d 100 Polyepoxide b-2a 75 75 75 75 75 b-2b 75 Crosslinking agentNM-20SE 75 75 75 75 75 43 BL-3175 75 Catalyst Dibutyltin dilaurate 2.5Coloring pigment Phthalocyanine blue 2.5 2.5 2.5 2.5 2.5 2.5 1.4 Flakyaluminum 0.5 0.5 0.5 0.5 0.5 0.5 0.3 Light transmittance (%) 45 45 45 4545 45 45

The compositions (B)-(1) to (B)-(4) are second coating compositionsaccording to the present invention, whereas the compositions (B)-(5) to(B)-(7) are for comparison.

“NM-20SE” and “BL-3175” shown in Table 1 are the following crosslinkingagents:

NM-20SE: a n-butylated melamine resin manufactured by Mitsui ChemicalCo., having a solid content of 60%

BL-3175: a hexamethylene diisocyanurate-based blocked isocyanatemanufactured by Sumitomo Bayer Urethane Co., Ltd., having a solidcontent of 75%

PRODUCTION EXAMPLE 9 Production of Powder Clear Coating (C)

A flask was charged with 40 parts of methyl methacrylate, 30 parts of2-ethylhexyl acrylate, 30 parts of glycidyl methacrylate, 10 parts ofstyrene, 1 part of t-butyl peroxide and 2 parts of oleic acid potashsoap (surfactant). The mixture in the flask was thermally polymerized bya suspension polymerization method and dried, giving aglycidyl-containing acrylic resin. The resin had a glass transitiontemperature of about 60° C.

100 parts of the acrylic resin, 25 parts of decamethylene dicarboxylicacid and 1 part of a surface modifier were melted and kneaded in aheating kneader, at 120° C. for 10 minutes. The knead was then cooledand grained using a grinder to thereby obtain a powder clear coatingcomposition (C-1) having a particle size of about to 150 μm.

PRODUCTION EXAMPLE 10 Production of Organic Solvent-based Clear CoatingComposition (C)

232 g of 2-hydroxyethyl acrylate, 72 g of acrylic acid, 546 g of n-butylmethacrylate, 150 g of styrene and 20 g of azobisisobutyronitrile werepolymerized in 1,000 g of xylene, giving a solution of a hydroxyl- andcarboxyl-containing acrylic resin (c-2a), having a resin solid contentof 50%. The resin (c-2a) had a number average molecular weight of20,000, a hydroxyl value of 112 mg KOH/g and an acid value of 56 mgKOH/g.

Separately, 392 g of 3,4-epoxycyclohexylmethyl methacrylate, 608 g of2-ethylhexyl methacrylate and 20 g of azobisisobutyronitrile werepolymerized in 1,000 g of xylene, giving a solution of anepoxy-containing acrylic resin (c-2b), having a resin solid content of50%. The resin (c-2b) had a number average molecular weight of 20,000and contained an average of 40 epoxy groups per molecule.

67 parts (as solids) of the resin (c-2a) and 33 parts (as solids) of theresin (C-2b) were mixed together, and the mixture was diluted with“Swasol 1000” (tradename, a hydrocarbon solvent manufactured by CosmoOil Co., Ltd.) to a viscosity of 20 seconds (Ford Cup #4/20° C.), tothereby obtain an organic solvent-based thermosetting clear coatingcomposition (C-2).

EXAMPLES 1 TO 8 AND COMPARATIVE EXAMPLES 1 TO 3

Substrate was prepared by applying, to metal plates (each 150×100×0.8mm), an epoxy cationic electrodeposition coating composition (tradename“Elecron #9400”, a product of Kansai Paint Co., Ltd.) and anintermediate coating composition (tradename “TP-37”, an organicsolvent-based polyester-melamine coating composition manufactured byKansai Paint Co., Ltd.), followed by thermal curing. The first coatingcomposition (A) was applied by air spraying to each substrate to athickness of 20 μm (when cured), and allowed to stand at roomtemperature for about 3 minutes. Then, the second coating composition(B) was applied to the uncured coating surface of the composition (A) byair spraying to a thickness of 20 μm (when cured), and allowed to standfor about 3 minutes. The two compositions were cured by heating at 140°C. for 30 minutes. Subsequently, the clear coating composition (C-1) ofProduction Example 9 or the clear coating composition (C-2) ofProduction Example 10 was applied to the cured coating surface. Whenusing the composition (C-1), the composition was applied byelectrostatic spraying to a thickness of 40 μm (when cured), and whenusing the composition (C-2), the composition was applied by air sprayingto a thickness of 40 μm (when cured). After being allowed to stand atroom temperature for 3 minutes, the clear coating composition isthermally cured under the baking conditions shown in Table 2.

The coated plates obtained by the above procedure were tested by thefollowing methods. Finished appearance: The coated plates were visuallyevaluated on the following scale. A: Good in gloss, smoothness and thelike; B: Notably inferior in gloss, smoothness and the like; C:Extremely inferior in gloss, smoothness and the like.

Initial intercoat adhesion: On the multilayer topcoat film of each ofthe coated plates immediately after preparation, 11 cuts reaching thesubstrate were made at 2 mm intervals using a cutter knife. Then, 11cuts perpendicular to the previously formed cuts were made in a similarmanner at 2 mm intervals, so as to form 100 squares (2×2 mm). At 20° C.,an adhesive cellophane tape was adhered to the cut surface and thenrapidly peeled off. The number of squares that remained on each coatedplate was counted to evaluate the initial intercoat adhesion on thefollowing scale. A: No square peeled off, showing excellent adhesion; B:1 to 10 squares peeled off, showing slightly inferior adhesion; C: 11 ormore squares peeled off, showing inferior adhesion.

Intercoat adhesion after water dipping: The coated plates were dipped inwater at 80° C. for 24 hours. Then, on the multilayer topcoat film ofeach coated plate, 11 cuts reaching the substrate were made at 2 mmintervals using a cutter knife. Subsequently, 11 cuts perpendicular tothe previously formed cuts were made in a similar manner at 2 mmintervals, so as to form 100 squares (2×2 mm). At 20° C., an adhesivecellophane tape was adhered to the cut surface and then rapidly peeledoff. The number of squares that remained on each coated plate wascounted to evaluate the intercoat adhesion after water dipping on thefollowing scale. A: No square peeled off, showing excellent adhesion; B:1 to 10 squares peeled off, showing slightly inferior adhesion; C: 11 ormore squares peeled off, showing inferior adhesion.

Table 2 shows the coating compositions used, conditions for baking theclear coating compositions, and results of the film performance test.

TABLE 2 Examples Comp. Ex. 1 2 3 4 5 6 7 8 1 2 3 First coating A-1 A-1A-1 composition (A) Second coating (1) (2) (3) (4) (1) (2) (3) (4) (5)(6) (7) composition (B) Clear coating C-1 C-2 C-2 composition (C)Conditions for 170° C. 140° C. 140° C. baking the 30 min 30 min 30 mincomposition (C) Film performance properties Finished A A A A A A A A A AA appearance Initial intercoat A A A A A A A A A A A adhesion IntercoatA A A A A A A A C C C adhesion after water dipping

What is claimed is:
 1. A 3-coat 2-bake method of forming a multilayered topcoat film, comprising applying a first coating composition (A) and a second coating composition (B) to a substrate, thermally curing the two compositions, and applying and thermally curing a clear coating composition (C); the first coating composition (A) being an organic solvent-based colored coating composition; the second coating composition (B) being an organic solvent-based coating composition comprising an acrylic resin (b-1) whose main chain has, bonded thereto, at least two side chains of different lengths each having at least one hydroxyl group, a polyepoxide (b-2) and a crosslinking agent (b-3); and the clear coating composition (C) being a powder coating composition (C-1) or an organic solvent-based coating composition (C-2) comprising a hydroxyl- and carboxyl-containing resin (c-2a) and a polyepoxide (C-2b).
 2. A method according to claim 1 wherein the first coating composition (A) has such a hiding power that a 15 μm thick cured coating of the composition (A) has a light transmittance of 3% or less in the wavelength range of 400 to 700 nm.
 3. A method according to claim 1 wherein the second coating composition (B) is a composition that forms a colorless or colored clear coating, the composition (B) having a such a transparency that a 15 μm thick cured coating of the composition (B) has a light transmittance of about 30 to 100% in the wavelength range of 400 to 700 nm.
 4. A method according to claim 1 wherein, of the at least two side chains bonded to the main chain of the acrylic resin (b-1), the difference in length between a side chain having a longer atomic chain between the hydroxyl group and the main chain, and a side chain having a shorter atomic chain between the hydroxyl group and the main chain, is 2 or more, in terms of the number of atoms constituting the side chains.
 5. A method according to claim 1 wherein the acrylic resin (b-1) has a hydroxyl value of 50 to 200 mg KOH/g and a number average molecular weight of 3,000 to 50,000.
 6. A method according to claim 1 wherein the powder coating composition (C-1) contains a glycidyl-containing acrylic resin and an aliphatic dibasic acid.
 7. A method according to claim 1 wherein the hydroxyl- and carboxyl-containing resin (c-2a) has an acid value of 15 to 150 mg KOH/g.
 8. A method according to claim 1 wherein the polyepoxide (c-2b) is a radical polymer containing an epoxy-containing ethylenically unsaturated monomer as a monomer component.
 9. A method according to claim 1 wherein the substrate is an automobile body. 